Ion chromatography is a common technique for analysis of sample materials. Conventional ion chromatography typically includes a chromatographic separation stage using an eluent containing an electrolyte and an eluent suppression stage followed by detection. In the chromatographic separation stage, analyte ions of interest in an injected sample are eluted through a separation column using an electrolyte as the eluent. In the suppression stage, electrical conductivity of the electrolyte is suppressed while not affecting the separated ions so that the ions may be determined by a conductivity cell. This technique is described in detail in U.S. Pat. Nos. 3,897,213; 3,920,397; 3,925,019 and 3,926,559.
Dilute solutions of acids, bases, or salts are commonly used as chromatographic eluents. Traditionally, these eluents are prepared off-line by dilution with reagent-grade chemicals. Off-line preparation of chromatographic eluents can be tedious and prone to operator errors, and often introduces contaminants. For example, dilute sodium hydroxide (NaOH) solutions, widely used as eluents in the ion chromatographic separation of anions, are easily contaminated by carbonate. The preparation of carbonate-free NaOH eluents is difficult because carbonate can be introduced as an impurity from the reagents or by adsorption of carbon dioxide from air. The presence of carbonate in NaOH eluents can compromise the performance of an ion chromatographic method, and can cause an undesirable chromatographic baseline drift during a hydroxide gradient and even irreproducible retention times of target analytes. In recent years, several approaches that utilize the electrolysis of water and charge-selective electromigration of ions through ion-exchange media have been investigated by researchers to purify or generate high-purity ion chromatographic eluents. U.S. Pat. Nos. 6,225,129, 6,682,701, and 6,955,922 describe electrolytic devices that can be used to generate high purity acid and base solutions by using water as the carrier. Using these devices, high purity, contaminant-free acid or base solutions are automatically generated on-line for use as eluents in chromatographic separations. These devices simplify gradient separations that can now be performed using electrical current gradients, with minimal delay, instead of using a conventional mechanical gradient pump.
With conventional electrolytic eluent generators, however, gases can be introduced into the eluent during the electrolytic reaction or at other stages in the analysis process. For example, in a large capacity potassium hydroxide (KOH) generator, electrolysis reactions produce hydrogen and oxygen gases. When used in a chromatography system, the hydrogen gas, along with the KOH solution, is carried forward into the chromatographic flow path. If hydrogen gas is produced in a significant volume relative to the liquid flow, its presence can be detrimental to the detection process and other downstream chromatography processes.
One solution to the problem of a presence of gas in the eluent is disclosed by U.S. Pat. No. 6,225,129 to Liu et al. (“Liu patent”). The Liu patent discloses a method for addressing the potential problem of hydrogen gas by application of Boyle's law. A flow restrictor is placed after the detector flow cell to create backpressure and elevate the pressure of the entire chromatography system. Under elevated pressure (e.g., 1000 psi or higher), hydrogen gas is compressed to an insignificant volume compared to the eluent flow so that it does not interfere with the downstream chromatography process. But this approach has several drawbacks. Because of the elevated pressures, the detector flow cell must be capable of withstanding a pressure of 1000 psi or more. In the case of ion chromatography system using suppressed conductivity detection, the suppressor must also be capable of withstanding an elevated high pressure. Therefore, this approach limits the type of components that can be used in an ion chromatography system employing an electrolytic eluent generator.
Another approach involves using an on-line gas removal device to remove hydrogen gas from the KOH solution. One way to remove the gas from an effluent is to pass the effluent through a gas removal device having a gas diffusion membrane prior to reaching the detection cell. An exemplar of a gas removal device used with a chromatography system is disclosed in U.S. Pat. No. 5,045,204 to Dasgupta et al. (“Dasgupta patent”).
The Dasgupta system includes a device for removal of gas (e.g. hydrogen) generated in the electrolytic cell from the product stream (e.g. sodium hydroxide). In one embodiment, the gas removal device is a gas diffusion cell including a plurality of blocks and a gas diffusion membrane separating a degassed product channel from a gas carrier channel. In another embodiment, gas-containing product is directed into a porous hydrophobic tube that is configured for the product to flow downwardly and then upwardly out of an exit port. The tube is formed of hydrophobic materials (e.g. as porous polytetraofluoroethylene (PTFE), (expanded)PTFE, Accurel®, or Celgard®) similar to the membrane. The hydrogen gas flows outwardly through the tube to a gas vent. As the KOH eluent stream passes through the tube under pressure, hydrogen gas diffuses through the tube and is carried to waste. In this manner gas is effectively removed from the eluent before it reaches the sample injector of the chromatography system so that the downstream chromatographic process is not affected. One advantage of this system is that a conventional detector flow cell and ion chromatography suppressor can be used.
The Liu patent discloses a similar gas removal device for on-line removal of gas from the eluent solution. The gas removal device includes a gas-permeable tubing coaxially aligned within a protective tubing. The gas-permeable tubing functions like a membrane. In operation, the KOH solution containing hydrogen gas is pumped through the gas permeable tubing and the hydrogen gas escapes through the tubing. A stream of aqueous solution flowing in an annular space between the outside of the gas permeable tubing and the protective tubing carries away the released gas.
One problem with such conventional gas removal devices is that current gas diffusion materials can not withstand pressures found in modern systems. Ion chromatography systems, in particular high-performance liquid chromatography (HPLC) systems, experience high in-line pressures. Conventional membrane materials have low burst pressures by comparison. By example, typical systems can rise above 1000 psi, and modern pumps can generate pressures in excess of 3000 psi and even 5000 psi. Such pressure levels are above the burst pressure of porous and gas-permeable tubing used for conventional gas removal devices such as those of the Dasgupta and Liu patents. Further, the low pressure threshold of such conventional devices limits the capabilities of the overall system. For example, systems making use of such gas removal devices are limited to about 3000 psi in the separation column. High pressure is desirable for greater efficiency and performance.
One solution to this has been to position the electrolytic eluent generator and the gas removal device on the low pressure side of the system, meaning in the pump intake line, or external (off-line) to the system. However, these positioning solutions limit the effectiveness of the devices and add to the volume of the electrolytic eluent generation system, thus compromising the overall performance of the ion chromatography system.
Thus, there is need to develop a degasser device that can be used in conjunction with an electrolytic eluent generator in ion chromatography and liquid chromatography systems over a wider range of operational pressures. There is a continuing need for chromatography systems with increased efficiency and performance.
In light of the foregoing, it would be beneficial to have methods and apparatuses which overcome the above and other disadvantages of known gas removal devices and chromatography systems.